Search

With the development of structural architectures, the demand for metamaterials with multiphysical characteristics has increased. The relationships between different properties, such as mechanical and mass-transport properties for bone scaffolds, are often mutually conflicting. Consequently, the optimization of these properties for metamaterials structured on the micro- to macroscale is challenging. In this study, inspired by the Hall–Petch relation, which indicates how one can independently tailor the strength and mass, we use a diamond configuration to construct microlattice metamaterials with decoupled mechanical and mass-transport properties to cater to artificial bone scaffolds. The elasticity and permeability are synchronously optimized using simple scaling laws. The results of compression experiments and transportation calculations demonstrate the promising performance of microlattices with an aspect ratio of 1 and at least 4 unit cells in the array direction. Furthermore, hierarchical microlattice metamaterials partitioned into spongy and compact zones are designed for bone scaffold applications. The proposed approach provides an innovative framework for developing multi-physics metamaterials for widespread engineering applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Matrix-body PDC bits endure the reactive shear force and impact damage during the rotary cutting of rock formations under downhole, making the bit blades vulnerable to premature breakage. To avoid the blade breakage, the key is to improve the fracture strength (also known as transverse rupture strength, TRS) of the matrix body of PDC bits. This work investigated the effects of size, shape, and type of tungsten carbides on fracture strength. Weibull distribution was adopted to describe the variations in the repeated TRS test results, which proved to be more appropriate than the normal distribution and could be used as an indirect indicator of the quality and performance of the matrix body. The results of this study showed that the median particle size is the predominant factor determining the TRS performance of the matrix body. Small-sized carbide particles deliver higher TRS, indicating that the fracture strength of the matrix body follows the Hall-Petch relation. Based on this finding, the fracture strength can be significantly reinforced to extend the service life of matrix-body PDC bits under harsh downhole conditions, leading to the reduction of trips and dramatic savings. However, the small-sized carbide particles are inherent lack of erosion resistance and accordingly limit their applications. • Fracture strength of the matrix body of PDC drill bits follows the Hall-Petch relation. • Small-sized tungsten carbide particles deliver higher fracture strength. • However, small carbide particles are inherent lack of erosion resistance and accordingly limit their applications. • Weibull distribution is more appropriate than normal distribution to represent strength variations in repeated tests. • The calculated Weibull modulus is a valuable indirect measurement of the infiltration quality and matrix performance. [ABSTRACT FROM AUTHOR]

A new physical model to explain the Hall–Petch relation is proposed. The main assumption is that the Hall-Petch slope (ky), is proportional to the work required to eject dislocations from grain boundaries. The model predicts a traditional Hall–Petch relation down to a grain size of about 5 μm, below which a deviation from linearity is observed. The strengthening peaks in the vicinity of 30 nm, and further grain refinement leads to softening. A semi-empirical equation is developed, which takes into account the effects of grain boundary strain energy. The model is applicable to both equiaxed and non-equiaxed grain structures. [Copyright &y& Elsevier]

Abstract: Characteristic grain growth occurred coincident with the Ostwald ripening of cementite particles upon annealing in a submicron-grained ferrite/cementite steel. The predictive capability of the Hall–Petch relation between the hardness and average ferrite size was demonstrated, independent of the distribution and size of cementite particles. [Copyright &y& Elsevier]

Inverse Hall-Petch behavior was observed in a transparent nanocrystalline magnesium aluminate spinel ceramic with grain size lower than 30 nm fabricated by high pressure spark plasma sintering (HPSPS). The ceramic was consolidated from 12 nm powder, synthesized by a solution combustion method and displayed an optical transmission of 80% at 550 nm wavelength. The hardness values of the specimens followed the Hall-Petch relation down to a grain size of about 30 nm, reaching a maximal hardness of 20 GPa. The hardness value decreased from 20 to 17.6 GPa when grain size decreased to 17 nm. [ABSTRACT FROM AUTHOR]

It has been shown experimentally that the Hall-Petch relation of nanocrystalline (NC) metal will be broken down when the average grain size (d) is below a critical value, but the mechanism behind this remains to be quantitatively analyzed. In order to capture the subtle evolution of grain boundaries (GBs), we recently developed a novel algorithm by using moving least-squares (MLS) interpolant. Moreover, the phase field model is used to build NC copper with more natural and physical GBs for molecular dynamics (MD) tensile simulation. The results show that the variation of stress at GBs (σ GB) and at grain interiors (σ GI) in the elastic stage are in good agreement with previous study. The σ GB exhibits periodic vibration, while σ GI shows a linear behavior. From the stress distribution, we find that the increased σ GB in one cycle can make the σ GI increase. The microplasticity of GBs occurs when σ GB increases to a peak value, which attenuates the stress concentration and then leads to the decrease of the σ GB. Therefore, the σ GI can be affected by the frequency of vibrated σ GB. The fast Fourier transform (FFT) results show that the dominant frequency for the model with d of 13.8 nm is larger than that for other models, which causes a larger Young's modulus in the model. The GBs and GIs supplement each other during deformation: GBs providing an extra stress to GIs, GIs supplying the space for microplasticity of GBs. Besides, the fraction of deformed GBs and rotated GBs in the model are also large. GB activities are the results of overall microplasticity before σ GB = σ GI and deformation during the plastic stage. The rotated GBs results in the emission of many Shockley partial dislocation (partials) from GBs since they create larger paths for dislocation movement. Thus, many twinning boundaries (TBs) are generated in the model with d of 13.8 nm by partials gliding on the successive plane of adjacent stacking faults (SFs) structures, which plays an important role in work hardening of NC Cu. Image 1 • A novel algorithm is compiled to quantitatively analyze GB activities. • NC Cu with more natural GBs is created by phase field for MD simulation. • GBs providing extra stress to GIs, GIs supplying space for microplasticity of GBs. • Young's modulus can be affected by dominant frequency of the vibrated σ GB. • Reverse Hall-Petch relation of flow-stress strongly depends on GBs activities. [ABSTRACT FROM AUTHOR]

Al–Mg–Sc alloys with fine equiaxed grain structure and free of Mg segregation and dislocations were fabricated by friction stir processing and post-solution treatment to study the deformation behavior of Al Mg base alloys at room and elevated temperatures. The k -value in the Hall–Petch equation of the alloys increased with increasing Mg content, and the equation of k -values and Mg content was displayed. Grain refinement and high Mg content not only could reduce the grain boundary (GB) sliding temperature of Al–Mg–Sc alloys during elevated-temperature deformation but also enhance their high-temperature GB sliding capacity. • k -value in H P equation of Al–Mg–Sc alloys increased with increasing Mg content. • k -value of Al–Mg–Sc alloys was developed in the form of k = − 80.56 × exp − c 1.94 + 148.2. • Grain refinement and high Mg content reduced the GB sliding temperature. • Grain refinement and high Mg content promoted the high-temperature GBs sliding. [ABSTRACT FROM AUTHOR]

The Hall–Petch relation is discussed separately for the yield stress of undeformed polycrystalline metals and for the flow stress of deformed metals. Key structural parameters are the boundary spacing, between grain boundaries in the former case and between dislocation boundaries and high angle boundaries in the latter. An analysis of experimental data supports the Hall–Petch relation for undeformed metals over a grain size range from about 20 nm to hundreds of micrometers. For deformed metals, boundary strengthening is not a constant and the Hall–Petch relation must be modified. [Copyright &y& Elsevier]

Deviation from the normal Hall–Petch relationship was observed in an ultrafine-grained Mg–Li alloy at low strain rates where plastic flow was governed by power-law creep. Based on the deformation analysis, the abnormal Hall–Petch relation for many metals often observed at nanograin sizes could be explained in terms of a transition from power-law breakdown to a grain-size-independent creep mechanism or a transition from a grain-size-independent to a grain-size-dependent mechanism, the condition of which depends greatly on strain rate and diffusivity. [Copyright &y& Elsevier]

Bata and Pereloma describe a physical model that proposes to explain the Hall–Petch relationship. The model is based on an averaging process for the work required to emit a dislocation from a tilt boundary. The purpose of the present comment is to examine several assumptions at the heart of the proposed model. Consideration is first directed at the fundamental premise that application of an average stress is sufficient to induce emission of a straight dislocation from a boundary, even though that stress is far below the stress required to overcome the highest energy barrier. The averaging process is also shown to emphasize the weak long-range interaction of the emitted dislocation with the original boundary, the consequences of which are considered. [Copyright &y& Elsevier]

Highlights: [•] Dislocation based continuum plasticity model incorporating Frank–Read sources. [•] Simple representation of densities of curved dislocations with their local geometry. [•] Derived from and validated by discrete dislocation model. [•] Grain aspect ratio dependent Hall–Petch relations in 2-dimensions are derived. [•] Predictions of our formulas agree excellently with discrete dislocation simulations. [ABSTRACT FROM AUTHOR]

Abstract: Experimental evidence that nitrogen does not segregate to grain boundaries in austenitic steels, presented as inconsistent with the model proposed by the authors, is examined. Thermodynamics indicate that segregation of nitrogen to grain boundaries is likely, especially at low temperatures. Solute nitrogen levels in question may have also been at or below the detection threshold of the instrument used. [Copyright &y& Elsevier]

Abstract: We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe90Ni10, Fe85Al4Si11, Ni99Fe1 and Ni90Fe10 powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal (T <0.4 T m) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall–Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing ∼1.5at.% impurities) and electrodeposited nanocrystalline Ni (containing ∼1.0at.% impurities). [Copyright &y& Elsevier]

Abstract: Bata and Pereloma developed a model for Hall–Petch relation based on the theory of grain boundary dislocation sources. Using experimental data on the effects of carbon and nitrogen on the Hall–Petch slope and their affinity to grain boundaries in austenitic steels, it is shown that the critical factor for the proposed model, grain boundary segregation, does not control the Hall–Petch slope. [Copyright &y& Elsevier]

Abstract: We report the morphology, texture, mechanical properties of Ni films with nanosized grains prepared by electrodeposition as a function of current density. With an increase in current density, the morphology of the nanostructured films was greatly affected and the textures changed from face-centered cubic (111)- to (200)-orientation. Consequently, the textural and microstructural alteration exerted strong influence on the hardness of the films, leading to decrease in the hardness (in particular, the inverse Hall–Petch relation) as current density increased, attributed to the distribution of colonies of grain agglomerate and the variation of directional tensile strength. [Copyright &y& Elsevier]

Understanding the effect of temperature variation on the microstructural evolution is particularly important to refractory high-entropy alloys (RHEAs), given their potential high-temperature applications. Here, we experimentally investigated the grain-growth behavior of the HfNbTaZrTi RHEAs during recrystallization at temperatures from 1,000 to 1,200 °C for varied durations, following cold rolling with a 70% thickness reduction. Following the classical grain-growth kinetics analysis, two activation energies are obtained: 205 kJ/mol between 1,000 and 1,100 °C, and 401 kJ/mol between 1,100 and 1,200 °C, which suggests two mechanisms of grain growth. Moreover, the yield strength – grain size relation was found to be well described by the Hall-Petch relation in the form of σ y = 942 + 270 D − 0.5 . It was revealed that the friction stress, 942 MPa, in the HfNbTaZrTi HEA is higher than that of tungsten alloys, which indicates the high intrinsic stress in the BCC-RHEA. The coefficient, 270 MPa/μm −1/2, is much lower than that in the 316 stainless steel and Al 0.3 CoCrFeNi HEAs, which indicates low grain-boundary strengthening. • HfNbTaTiZr HEA with a 70% cold-rolled reduction was annealed from 1000 from 1200 °C. • Grain-growth exponent and activation energy were obtained by grain-growth kinetic. • Yield strength – grain size relation was described by the Hall-Petch relation. • A high friction stress is found in the HfNbTaZrTi HEA. [ABSTRACT FROM AUTHOR]

Abstract Zirconium carbide (ZrC x) is an important structural material. Its high strength and hardness makes it suitable for application in hard materials and non-oxide type new ceramics. Hence, several researches have been carried out with the aim of understanding its mechanical properties. In this paper, the mechanical properties of ZrC x (0.45 ≤ x ≤ 0.72) were investigated considering the grain size effect. The bending strength as a function of grain size obeyed the Hall–Petch relation. The composition dependence of the strength linearly decreased with decreasing carbon content, followed by saturation at x = 0.6, corresponding to the lowest x value of the ZrC x single phase region. The Young's modulus and hardness monotonically decreased with decreasing carbon content, independently of grain size. The annealing of ZrC 0.45 at 1273 K for 18 ks led to the formation of Zr precipitates, which showed an orientation relationship with the ZrC matrix of (0001) Zr //(111) ZrC. These precipitations formed compressive residual stress in the ZrC phase, resulting in an 80% increase in the bending strength after annealing. Graphical abstract fx1 Highlights • Effect of composition and grain size on mechanical properties of ZrC x investigated. • Bending strength obeyed the Hall–Petch relation. • Young's modulus and hardness were independent of grain size. • Young's modulus and hardness monotonically decreased with decreasing carbon content. • Annealing of ZrC 0.45 formed coherent Zr precipitates, improving bending strength. [ABSTRACT FROM AUTHOR]

The length-scale effect on the hardness of Al/Si 3 N 4 multilayered composites were investigated via combined experimental and machine learning approaches. It was found that the hardness decreases with increase of the individual layer thickness, in agreement with the classic Hall-Petch relation. However, the hardness decrease became moderate when the individual layer thickness was less than 80 nm. The individual critical layer thickness of metallic/ceramic multilayered composites for the change in deformation mechanism was obtained by machine learning methods. The prediction results indicated that the individual critical transition layer thickness was mainly determined by the metal layer, and the corresponding critical transition hardness was positively correlated with the elastic modulus of the ceramic layer. Importantly, it was confirmed that the hardness of other metallic/ceramic multilayered composite systems could be predicted by the machine learning approach, especially the optimized random forest regression (RFR) model. [Display omitted] [ABSTRACT FROM AUTHOR]

We investigated the Hall-Petch relations in Al-ZnO composites with different matrix grain size ( D ) relative to interparticle spacing ( L ). For samples with D  >  L , it was found that grain refinement as described by Hall-Petch relation and particle strengthening by Orowan mechanism both significantly contribute to the strengthening of the composites. For samples with D  ≤  L , variation of microhardness with grain size was found to follow a good Hall-Petch relation with an enhanced Hall-Petch slope. Our study indicates the different coupling of strengthening mechanisms in metal matrix composites with different microstructure length scales. [ABSTRACT FROM AUTHOR]

The three questions posed by Wert, regarding the assumptions made in the derivation of the model proposed by the authors, are addressed. The low dependence of the Hall–Petch slope upon temperature supports the averaging approach used by the authors in the original paper. The effect of in-grain dislocations is not considered, because it is outside the assumptions of the proposed model. The effect of grain boundary defects is acknowledged, but based on experimental evidence not considered significant. [Copyright &y& Elsevier]

To get a range of crystallite size, 5 N pure polycrystalline zinc specimens were shined with 50–100 Nd:YAG laser shots. Williamson-Hall, Size-Strain Plot, Halder-Wagner, and Wagner-Aqua models were employed to evaluate the crystallite size and lattice strain of pristine and laser-treated specimens. Surface hardness of the specimens were measured under 0.2 and 0.3 kgf indentation loads for 10 s. In each case, the surface hardness followed inverse Hall – Petch Relation. For a fixed number of laser shots, the surface hardness was greater for 0.2 kgf indentation load than that for 0.3 kgf. The surface hardness of pristine zinc specimen increased on treatment with 50 laser shots. With further rise in laser shots up to 80, the surface hardness progressively decreased close to that of pristine specimen. Later, it again increased for 90 laser shots and then slightly decreased for 100 laser shots. • 5 N pure zinc specimens were shined with 50–100 Nd:YAG laser shots. • Williamson-Hall, Size-Strain, Halder-Wagner, Wagner-Aqua models provided structural parameters. • Surface hardness was measured under 0.2 and 0.3 kgf indentation loads for 10 s. • Surface hardness followed inverse Hall – Petch Relation. • Surface hardness was greater for 0.2 kgf indentation load than that for 0.3 kgf. [ABSTRACT FROM AUTHOR]

This study examined the effect of grain size on the tensile properties in {332}<113> twinning-induced plasticity (TWIP) Ti–15Mo alloy and Ti–15Mo–1Fe alloy deformed by dislocation slip for comparison. At the yield point, a large Hall-Petch coefficient was observed for {332}<113> twinning, which was not only due to the increased stress concentration from the interaction between the activated twins before the macroscopic yield and the grain boundary, but also a larger twin volume for macroscopic yield in the coarse-grained Ti–15Mo alloy. It gradually decreased with increasing strain, and negative values were obtained at strains above 0.23. The dynamic Hall-Petch relation was established by the effective grain size, which was similar at strains of up to 0.20 in Ti–15Mo alloy with static grain sizes of 29 μm and 108 μm. The decreasingly modified Hall-Petch coefficient was resulted from the successive occurrence of twinning activation, twinning suppression, and the twin-dislocation and dislocation-dislocation interactions. • The effect of grain size on tensile properties is studied in TWIP Ti–Mo alloy. • The large static grain size dependency for {332}<113> twinning is clarified. • The inverse Hall-Petch relation at large strains is observed in TWIP Ti–15Mo alloy. • The dynamic Hall-Petch relation is established by effective grain size. • Grain refinement is a method to improve strength-ductility balance in TWIP Ti alloys. [ABSTRACT FROM AUTHOR]

This study focuses on the role of pyramidal 〈 c + a 〉 dislocations in the grain refinement mechanism in the Ti-6Al-4V alloy with an initial { 11 2 ¯ 0 } < 10 1 ¯ 0 > rolling texture. A large number of pyramidal 〈 c + a 〉 dislocations were activated in the sample subjected to the severe shot peening process. Two important roles of pyramidal 〈 c + a 〉 dislocations were discovered. First, pyramidal 〈 c + a 〉 slip coordinates the large c -axis strain, thereby achieving generalized plastic flow, especially in nanograins. Second, the unique low-angle grain boundaries (LAGBs) with basal-pyramidal dislocation locks (prismatic 〈 c 〉 and prismatic 〈 c + a 〉 dislocations) were produced for the first time by pyramidal 〈 c + a 〉 interacting with basal 〈 a 〉 dislocations. This unique low-energy boundary greatly enhances the stability of the strain-induced grain boundary and dislocation density (~6.6 × 1015 m −2 in nanograins). The grain refinement process contains three types of subdivision modes: (I) dislocation walls with pyramidal 〈 c + a 〉 dislocations in coarse grains; (II) basal 〈 a 〉 intersecting with prismatic 〈 a 〉 dislocations in coarse grains; and (III) basal 〈 a 〉 intersecting with pyramidal 〈 c + a 〉 dislocations in coarse grains, ultrafine-grains and nanograins. The occurrence of slip modes depends on the initial texture and texture evolution during dynamic recrystallization. Besides, Hall-Petch breakdown at the nanoscale was found and is attributed to the decreasing critical resolved shear stress of pyramidal 〈 c + a 〉 slip at the nanoscale. This study provides a new approach for the design of stable nanostructured hexagonal close-packed metals by the unique LAGBs with basal-pyramidal dislocation locks. Image, graphical abstract [ABSTRACT FROM AUTHOR]

A molecular dynamics simulation study has been carried out to clarify the effect of grain size on the deformation behavior of nanocrystalline body-centered cubic Fe. Average flow stresses were found to decrease with grain refinement below 14.7nm, exhibiting a breakdown in the Hall–Petch relation. A change in the dominant deformation mechanism from dislocation glide to grain boundary sliding appeared to be the direct cause of the breakdown in the Hall–Petch relation observed in the present nanocrystalline pure Fe. [ABSTRACT FROM AUTHOR]

The nanoindentation process of single-crystal and nanocrystalline copper is studied with molecular dynamics simulations based on the many-body tight-binding potential. The grain size effect is evaluated in terms of slip vector, atomic stress, loading force, and hardness. The inverse Hall–Petch relation is found below 7 nm. With grain size smaller than 5 nm, the equivalent stress decreases quickly and stress-induced grain growth is observed in indentation process. Grain rotation is the main cause of grain coarsening for small grain sizes. For larger grain sizes, dislocations are found at the {1 1 1} close-packed plane and {1 0 0} plane. [ABSTRACT FROM AUTHOR]

The high texture dependence of a Hall–Petch slope (k) for Mg alloys has been frequently reported. Several important equations used to calculate k have been previously developed, and although they seem to work well for fc c and bcc materials, they often fail to predict the highly texture-dependent k in Mg alloys. A new equation based on the dislocation pile-up model was developed in this study. The validity of this new equation was tested through a comparison of the predicted k values with the experimental values as well as the calculations from older equations. The results indicate that the new equation can achieve an accurate prediction for several previously reported texture effects on k , whereas the k values predicted by the older equations often exhibit a clear deviation. The reasons for this were analyzed and discussed. The strong deformation anisotropy for Mg alloys leads to a complex texture effect on k , including the effects from both external and internal stresses. Both effects are well expressed in the new equation. In contrast, the old equations consider the external stress effect, but do not express well the internal stress effect. In addition, the old equations consider only the predominant deformation mode. However, our results indicate that the activation of a portion of another deformation mode other than the predominant one plays an important role in the k value. In the new equation, all possible deformation modes and their fractions are considered in the calculation. Using the important parameters of the new equation, the mechanisms for several texture effects on k as previously reported were discussed and new understandings were obtained. Image 1 [ABSTRACT FROM AUTHOR]

In this work, we investigate the tensile strength and ductility of bulk Cu/Nb nanolaminates after exposure to high temperatures. We show that the interface transforms from flat to wavy at a transition temperature of 700 °C, and tensile strength is linearly proportional to H-1/2 (H = layer thickness). Moreover, the wavy interfaces give rise to a higher slope of the Hall-Petch law. This result can be attributed to greater resistance to slip transmission across wavy interfaces compared to planar interfaces. After 1000 °C for 1 h, the material still exhibits a high strength of 468 MPa and enhanced elongation. Unlabelled Image [ABSTRACT FROM AUTHOR]

[Display omitted] • MD is used to study the tensile, and fracture mechanics of NC 2D SiC. • Effects of grain size on the stress–strain profile of the NC-SiC have been investigated. • The NC-SiC encounters a substantial degradation in mechanical properties relative to its single-crystal counterpart. • Fracture strength as a function of grain size can be characterized by the inverse pseudo Hall-Petch relation. • Increasing grain size brings about more elasticity in the structure, albeit at the price of fracture strain. Two-dimensional Silicon Carbide (SiC) has opened the route to a cornucopia of advanced functionalities in the realm of quantum condensed matter. It holds great promise for highly efficient nanoelectronic, optoelectronic, renewable energy, and spintronic applications thanks to the confluence of a wide spectrum of mesmerizing physical properties like a wide direct bandgap with high exciton binding energy, robust spin–orbit-coupling, excellent photoluminescence, suitable mechanical strength, and thermodynamic stability. Nonetheless, it is still a daunting challenge to incorporate SiC in functional systems since extensive analyses of the mechanical properties, and fracture mechanism of nanocrystalline (NC)-SiC is still obscure. In this light, this work is an attempt to report detailed information concerning the room-temperature tensile mechanical properties and fracture phenomena of NC-SiC executing Molecular Dynamics (MD) simulations. In particular, effects of grain size on the stress–strain profile, fracture strength, fracture strain, and Young's modulus of the NC-SiC have been thoroughly investigated. It has been found that the strength as a function of grain size can be characterized by the inverse pseudo Hall-Petch relation. Increasing grain size brings about more elasticity in the structure, albeit at the price of fracture strain. The NC-SiC encounters a substantial degradation in mechanical properties relative to its single-crystal counterpart. Afterward, we performed an exhaustive fracture analysis on two NC-SiC samples of different grain sizes. The single-crystal SiC can endure more tensile strain before rupture compared to that of the NC-SiC. At last, the nanosheet exhibits more immunity to fracture with decreasing grain size. This study would lay the groundwork for NC-SiC to be successfully realized in functional systems as well as serving as a solid roadmap for engineering the mechanical properties of nanocrystalline materials. [ABSTRACT FROM AUTHOR]

• Grain refining increased yield stress by 100 % in the Cr 40 Co 40 Ni 20 alloy. • TRIP effect follows the Hall-Petch relation: σ TRIP = σ TRIP, 0 + k TRIP.d-0.5. • Hall-Petch slope is similar for both yield stress and TRIP effect (k y ≈ k TRIP). • Model correlating applied stress, transformed phase fraction, and FCC grain/crystallite size. A Cr 40 Co 40 Ni 20 (at.%) alloy with different grain/crystallite sizes was analyzed through in-situ synchrotron X-ray diffraction during tensile testing. The FCC starting structure underwent a partial strain-induced transformation to HCP (TRIP effect) and the percent transformed was measured throughout the deformation. The critical stress required to form a certain HCP fraction was shown to follow a Hall-Petch relation (σ TRIP = σ TRIP, 0 + k TRIP d-0.5), with the Hall-Petch slope being approximately the same for yield stress and TRIP effect (k y ≈ k TRIP). Furthermore, this work developed a Hall-Petch-based model that correlates the applied stress, the transformed phase fraction, and the initial FCC grain/crystallite size. It predicts the stress required to form a certain HCP fraction, or the fraction formed when a certain stress is applied, for different grain/crystallite sizes. We also proposed a mechanism to explain the grain/crystallite size dependence of the TRIP effect and discuss how the TRIP effect and its early activation in the Cr 40 Co 40 Ni 20 alloy provide high work-hardening capacity, which improves ductility and toughness. Here, a refined FCC grain size (d = 1.3; c = 0.7 μm) was shown to increase the yield stress by at least 100 % (417 → 834 MPa), compared to a coarser grain material (17; 6.8 μm), while maintaining a high ductility of 41 %. This work contributes to a better understanding of the deformation mechanisms, mainly the strain-induced phase transformation (TRIP), highlighting their impact and importance on mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The present work investigates Ni–nanodiamond and Ni–graphite composites produced by mechanical synthesis and subsequent heat treatments. Processing of nickel–carbon nanocomposites by this powder metallurgy route poses specific challenges, as carbon phases are prone to carbide conversion and amorphization. The processing window for carbide prevention has been established through X-ray diffraction by a systematic variation of the milling parameters. Transmission electron microscopy confirmed the absence of carbide and showed homogeneous particle distributions, as well as intimate bonding between the metallic matrix and the carbon phases. Ring diffraction patterns of chemically extracted carbon phases demonstrated that milled nanodiamond preserved crystallinity, while an essentially amorphous nature could be inferred for milled graphite. Raman spectra confirmed that nanodiamond particles remained largely unaffected by mechanical synthesis, whereas the bands of milled graphite were significantly changed into the typical amorphous carbon fingerprint. The results on the annealed nanocomposites showed that milling with Ni accelerated graphitization of the carbon phases during heat treatments at 973 and 1073K in both composites. At the finer scales, the nanocomposites exhibited a remarkable microhardness enhancement (∼70%) compared with pure nanostructured nickel. The Hall–Petch relation and the Orowan–Ashby equation are used to discuss strengthening mechanisms and the load transfer ability to the reinforcing particles. [Copyright &y& Elsevier]